An interview with a writer as artful as Javier Cercas is filled with traps for the unwary, beset by pitfalls for the unprepared. The literary interview is a territory the Spanish novelist explores at length in his international bestseller, Soldiers of Salamis, in which he drives the plot with a pair of fictional interviews considerably more colourful than many of their real-life counterparts.

The story, which won the Independent foreign fiction prize in 2004, begins when the narrator, a journalist and author named Javier Cercas, stumbles across the story of a minor poet's brush with death in the Spanish civil war during the course of an extended interview with a garrulous and evasive Spanish novelist - an interview which the fictional Cercas finally manages to "salvage", or perhaps makes up. The climactic third section of this "true tale" is set in motion by an interview with a bohemian Chilean novelist, who first tells the narrator that he doesn't need any imagination to write a novel, and later tells him to "make up" an encounter with the book's central figure - a tactic which the fictional Cercas rejects.

Meanwhile, the real Javier Cercas's latest book, The Speed of Light, follows an unnamed novelist, who has recently found success with a "true tale" about the Spanish civil war, as he grapples with the story of a Vietnam veteran whom he met while teaching at the University of Illinois - where Cercas himself spent two years in the 1980s. In the world of Javier Cercas, fact and fiction are never far apart, although this interview is at least grounded to some pretty tangible details: across the table from me is a short, compact man with jet black hair in a leather jacket. He seems a little on edge, but I presume it's the real him.

His self-reflexive technique, he explains, came out of a series of experimental columns for the Spanish newspaper, El Pais, which continues to this day. "I began to write some weird stuff in El Pais, using the 'I'," he says, "and then I became aware that this 'I' was fictional, even in a newspaper. They were experimental, crazy columns, and I began to write in a different way, that some people describe as 'self-fiction'."

So the books aren't true tales? "Of course not," he smiles. "These narrators in the books are not myself, even though in the case of Soldiers of Salamis the name is my name."

According to Cercas, the first person voice is the natural starting point for fiction. To reach the third person was a "conquest" for him, but having achieved that stage of detachment he realised that the first person offers the writer many possibilities.

"The idea of putting oneself in the novel, more or less explicitly, is not new," he observes. "There's a whole bunch of writers that use this kind of 'I'." He mentions Cervantes, Borges, Roth. "It's the same thing in poems: the 'I' that speaks in a poem by Auden is not Auden himself. It's a trick that he uses to say what he wants to say. It's a fiction - although this fiction can be truer than his true self ... it's like a mask."

As Rodney, the Vietnam veteran at the centre of The Speed of Light says, "talking a lot about oneself is the best way of hiding". It's an authorial strategy that makes the writer's appearances at readings or literary festivals fraught with opportunities for misunderstandings. Cercas says he gets mistaken for his narrators all the time. After the success of Soldiers of Salamis, he tells me, people would stop his father in the street and say "I thought you were dead", because the fictional Cercas had lost his father.

According to Cercas, there is only one "absolutely fictional character" in Soldiers of Salamis: the narrator's girlfriend, a fortuneteller named Conchi. But even here, the line between fact and fiction is obscure. "There is a fortuneteller in Girano, which is where the action takes place," says Cercas, "who thought she was Conchi. And she sued me." Luckily the judge accepted Cercas's argument that Soldiers of Salamis was a work of fiction, and the case was thrown out. "It's a risky thing to write novels, because you're working with your life, and with the lives of people that live around you."

If Rodney is right, however, and "the only stories worth telling are the true stories", then perhaps these misunderstandings are an indication that Cercas's strategy is successful.

"Maybe I began to write this way out of a dissatisfaction with pure fiction," he suggests. "[Walter] Benjamin talks about the crisis of fiction: how to convince people that something that is fictional, and that we know is fictional, is real. That's the main purpose of fiction." In response to this, Cercas began using historical or autobiographical material as the basis for fiction as a kind of trick. "I was trying to convince the reader," he says, " and trying to convince myself."

It was only in the middle of writing Soldiers of Salamis that he began to realise he needed to pursue the illusion of reality even further, and give the narrator his name. "In the middle of the book it became clear," he explains. "Everybody in the book had their real name - it's a sort of chronicle - so the narrator had to be myself in a sense, because everything else was real. Of course everything is fiction, in fact, but everything is supposed to be real, so the narrator should be myself."

With this head-spinning strategy in place, the structure of the book became clear, and he finished the book quickly, leaving himself free to return to a story he'd been trying to tell since his return from the US in the 1980s. "I wanted to write The Speed of Light before Soldiers," he says. "In fact I tried to write it when I came back from the US ... I tried and I tried, but I couldn't find a way to do it. But then when I finished Soldiers I thought 'OK, I know how to write it'."

There's a moment in The Speed of Light that shows the raw power of Cercas's blurring of fact and fiction. When a catastrophe strikes the narrator's wife and son, the reader is left gasping, unsure for a moment how to respond. Apparently Cercas's own wife read the passage herself without quaking - exhibiting a fortitude in the face of fictional calamity the author puts down to her theatrical experience - but Cercas admits that it was a difficult moment to write. "I'm not a brave person in my life," he says, "but when you sit in front of the screen, you have to be brave." The disaster that surrounds the narrator's success is, he says, like an "exorcism"; a way of keeping a similar fate at bay.

The kernel at the heart of The Speed of Light, the grain around which it is constructed, is a scene where Rodney, who has witnessed terrible things in Vietnam and perhaps taken part in a massacre, sits on a bench in a park for hours, watching children playing frisbee.

"I try to ask a question when I write a novel," states Cercas. "When I wrote Soldiers I wondered, 'what was this guy thinking about when he decided not to kill this other guy?'. With The Speed of Light I tried to ask 'what is he thinking, this guy who's looking at these kids playing, this guy who's seen these awful things?'."

He insists, however, that just as Soldiers of Salamis is not about the Spanish civil war, The Speed of Light is not about Vietnam. "Of course I was interested in Vietnam, I was interested in the Spanish civil war," he concedes, "but I was more interested in formulating these questions. And to ask them I needed to know about Vietnam, to understand the civil war. But the war wasn't the main point."

Despite the similarities between the mechanisms of the two books, it wasn't until after The Speed of Light was published that Cercas began thinking of them as a pair, when a review in the TLS described them as a "diptych". Now he believes that the books are almost mirror images of each other; like shadow and light. "Soldiers talks about something wonderful," he continues, "about the possibility of being noble and right ... The Speed of Light considers the opposite; the fact that given certain circumstances - and war is the worst circumstances of all - normal, generous intelligent people can become monsters."

The novels' narrators are also in some sense opposites - while the journalist in Soldiers of Salamis is trying to discover the "truth", the novelist in The Speed of Light is engaged in a process of self-justification. But perhaps the thing that separates the two books most clearly for Cercas is the difference in generation. He describes the Spanish civil war as a "far-away" story, "even though it's my country": the survivors are all in their 70s and 80s. "But Vietnam is my generation," he says. " I'm 45, so they're my older brothers - it's really close to me. Rodney is a fictionalisation of a real character; he could have been me ... he's 'uno de los nuestros' - one of ours."

With the US currently engaged in an ill-starred adventure in Iraq, writing about the Vietnam war has a political dimension that Cercas does not attempt to deny. The book will be published in the America later this year - a promotional tour the author prefers to avoid.

"I wouldn't feel comfortable going to the US," he explains, "because this is not a book about Vietnam, and if I went there now, that's what it would become."

He's also anxious to avoid the trap of repeating the same formula. When the author steps inside the novel there's a tendency which mirrors the escalation of invasiveness found in reality TV to "want more and more and more".

"There is an obvious similarity between my last two books," he explains, "and now I'm trying to get out of this. I'm looking for something, but I don't know what it is. I'm in crisis as a novelist, but I hope this crisis is good. If you're not in crisis, then you're dead, or" - he smiles - "you're repeating yourself."